Most of the currently available burglar alarms include a horn output line, which is connected to a siren horn for the latter to make a loud sound as a warning signal. To install the burglar alarm on a car, it is necessary to extend the horn output line from an interior of the car into an engine chamber via a firewall, and finally connect the horn output line to the siren horn. To enable successful extending of the horn output line through the firewall, it is necessary to expand a wiring hole and/or an engine hood opening stay hole originally provided on the firewall. Sometimes, it is even necessary to drill a new hole on the firewall to complete the installation of the burglar alarm. The expanding and drilling of holes on the firewall not only requires additional time and labor, but also spoils or damages the original airtight and watertight ability of the firewall to disadvantageously admit exhaust and wastewater to the car.
A solution to the problems of additional installing time and labor as well as spoiling the original airtight and watertight ability of the firewall is to design the siren horn as a wireless component for wirelessly transmitting warning signals. It is possible to control the wireless siren horn via a main unit of the burglar alarm simply by electrically connecting the siren horn to positive and negative electrodes of the battery of the car.
Moreover, to avoid mutual interference between the wireless siren horn and the main unit of the burglar alarm, the siren horn and the main unit must be designed to work at considerably different radio frequencies.
However, in some countries, only a certain band or a certain frequency is allowed for the purpose of a wireless burglar alarm. In this case, the wireless siren and the main unit of the burglar alarm having the same frequency must be actuated at different time to avoid interference with each other. A serious problem with the latter case is the siren horn could not operate as soon as an instruction is received from the main unit and therefore has largely reduced function.
Another way to overcome the above-mentioned problems is to use a power cord to transmit codes for the siren horn to make warning sounds. However, the power cord also carries noises that cause deformation in the magnitude of high and low voltages and therefore prevent the high and low voltages from being used as code 1 (one) and code 0 (zero), respectively.
It is therefore desirable to develop a burglar alarm using a power cord to transmit signals and a method for controlling the burglar alarm.